This invention relates to circuitry that provides a reference voltage, and more particularly relates to reference voltage circuitry that is implemented in integrated circuitry.
In general, voltage detection circuitry prevents utilization circuitry (e.g., memory) from operating when the power supply voltage is too low for proper operation. At power-up (e.g., start up) the voltage detection circuitry suppresses operation of the utilization circuitry until the supply voltage reaches a predetermined voltage. When the supply voltage reaches the predetermined voltage, the voltage detection circuitry may enable the utilization circuitry by asserting, for example, a POWER-UP ENABLE signal. Likewise, during power-down, the voltage detect circuitry can de-assert the POWER-UP ENABLE signal when the supply voltage falls below the predetermined voltage, thereby disabling the utilization circuitry.
The point at which the voltage detection circuitry changes the state of the POWER-UP ENABLE signal is sometimes referred to as the trip-point. During power-up (sometimes referred to herein as ramp-up), the trip-point occurs when the supply voltage exceeds a predetermined voltage. Similarly, during power-down (sometimes referred to herein as ramp-down), the trip-point occurs when the supply voltage falls below the predetermined voltage.
In conventional voltage detection circuitry, such as that shown FIG. 1, the trip-point corresponds to when transistor 120 turns ON and OFF. For example, during power-up, the POWER-UP ENABLE signal goes HIGH when transistor 120 turns ON. POWER-UP ENABLE goes high because the voltage at the drain of transistor 120 (Node A) is pulled to ground when transistor 120 is turned ON. This LOW signal is then inverted by inverter 130 to provide a HIGH POWER-UP ENABLE signal.
A problem with circuitry 100 (of FIG. 1) is that the point at which transistor 120 turns ON is subject to temperature and process variations. Process variation refers to the variance of one circuit to the next. For example, when transistors are fabricated, the threshold voltage may not be uniform for all transistors. One threshold voltage may be 0.7 volts, whereas another threshold voltage may be 0.8 volts. Other components, such as resistors, typically vary in resistance and equivalent series inductance. Thus, when circuitry 100 is constructed, the turn ON point of transistor 120 may vary from one circuit to the next. Moreover, changes in temperature can cause the trip-point of a particular circuit to vary. FIG. 2 shows how trip-points can vary from one voltage detection circuit to another and how temperature changes can affect the trip point for a particular voltage detection circuit.
This gross variance in trip-points is undesirable and can potentially result in permanent damage to utilization circuitry. For example, if the trip-point occurs before the supply voltage reaches a predetermined voltage, this may force the utilization circuitry to draw excessive current to compensate for being enabled at too low a voltage, potentially resulting in a circuit damaging current spike.
Therefore, it is an object of the invention to provide voltage detection circuitry that is insensitive to process and temperature variation.
It is also an object of the invention to provide voltage detection circuitry that has a substantially constant trip-point.